In modern digital transmission systems, a data signal is converted into a series of 0's and 1's called “bits”. In an ideal system, all the data signal bits that are sent down a transmission channel or transmission line have exactly the same length and spacing. This is important at the receiver end of the transmission channel so that the stream of 1's and 0's can be converted back into the original data signal.
Unfortunately, numerous factors in the transmission channel can alter or interfere with the desired precise timing of the data signal bits that are sent or transmitted. This applies to virtually all types of data communication, including telephone lines, computer networks, optical fibers, radio communication, and so forth. The resulting random variations in the timing of the signal are called “jitter”.
“Jitter measurement” refers to analyzing the variations in the timing of the bits and determining the nature and the amount of the timing uncertainty—or jitter—that has been put onto the data bits by the time that the data signals get to the receiving end of the transmission line. The measurements reveal both the amplitude of the jitter and the frequency of the jitter.
The amplitude of the jitter is the amount or size of the timing error in each of the bits. That is, jitter amplitude is the difference between the time that the bit should have arrived and the time that the bit actually does arrive.
The frequency of the jitter is a measurement that tells how quickly or how slowly the amount of jitter is changing. Thus, for example, the frequency of the jitter is a measurement of how quickly the bits switch back-and-forth from being early at one moment, then late at another, then early again, and so forth. Jitter frequency is thus the frequency of the variations in the timing of the data bits.
The jitter frequency, of course, is different from the frequency of the actual data bits in the transmission channel. The jitter frequency is normally from about 10 Hz up to around several percent of the data bit frequency rate in the transmission channel. The required bandwidth for jitter measurement, therefore, can be very large. For example, one specification for measuring jitter in a 155 Mb per second signal would require the ability to measure jitter up to 1.3 MHz.
As indicated, there are many sources of jitter. One source is “data-related” jitter. With data-related jitter, the jitter is associated with the non-repetitive nature of the string of 1's and 0's in a data signal. For example, if a long string of 0's is followed by a long string of 1's, or vice versa, the result can be a slight, instantaneous transition point in the data signal timing. This can be caused by many factors, such as power supply noise on the transmitter, crosstalk from other signals, relays, reflections in the transmission line, and so forth.
Other forms of jitter can appear within a multiplexed signal. One of these comes from combining separate source data signals into a single multiplexed signal. For example, in a 155 Mb/s multiplexed signal there might be 63 T1 signals, each at a frequency of 1.5 MHz. (The term “T1” generally refers to a high-speed data circuit line rate format that carries 24 user channels at a combined speed of 1.5 MHz.) The timings of these different signal bits from the various different T1 signals may not be equally spaced within the multiplexed 155 Mb/s channel. The different timing spacings then appear as jitter at the receiving end of the data channel.
There is a relationship between the jitter frequency and the difficulty of compensating for the jitter. Generally, the higher the jitter frequency, the smaller the jitter amplitude that can be managed. It is thus important to be able to measure both the amplitude of the jitter and the frequency (or frequency band) of the jitter. It would be particularly useful to be able to measure the spectrum of the jitter as well.
Traditional jitter measurement has been performed with analog circuitry. Such analog circuitry, unfortunately, has numerous shortcomings. For example, it is susceptible to signal noise, temperature variations, power supply noise, calibration problems, and so forth.
Analog circuits also become more challenging with increasing network and data system bit rate speeds. It is very difficult to get analog circuitry to function satisfactorily at high speeds and frequencies.
High frequency analog analysis circuitry is also difficult to miniaturize for portable use in analyzing data transmission lines in the field. It is also difficult to keep such portable analog circuitry calibrated and stable during the jostling and the temperature variations that occur as it is moved from place to place in the field.
Still another limitation with analog equipment is power consumption, particularly since oscillator power consumption increases as frequencies increase. Similarly, the need for shielding increases as frequencies increase.
All this results, typically, in a larger piece of equipment that might be acceptable in a laboratory environment but not in mobile testing equipment, and particularly not in handheld equipment intended for field use.
In addition to such limitations as increased power consumption, increased shielding, increased instability, and increased size, there are additional technical problems from increasing the speed of analog circuitry. These contribute as well to accuracy problems. For example, every tiny resistor and capacitor in an analog circuit has a manufacturing tolerance (typically from 1% to 10%). All these tolerances have to be taken into account when the circuit is designed, and sufficient calibration capacity must be incorporated to compensate for all the tolerance variations. Further, such devices have values that drift over time as well as with temperature. Thus, factory recalibration can sometimes be necessary as often as every six months.
Thus, a need still remains for improved jitter measurement methods and apparatus. In view of the continuing increase in data rates, transmission frequencies, and component miniaturization, it is increasingly critical that answers be found to these problems.
Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.